1. EE 4BD4 Lecture 16 Piezoelectric Transducers and Application 1

2. Piezoelectricity Theory Reversible system (Strain Electrical Potential) Asymmetrical crystal lattice with distortion producing equal and opposite charge on each surface Total induced charge q = kf where f = force in Newtons, k = piezoelectric constant Coulombs/Newton 2

3. Piezoelectric Materials Typical values for k are 2.3 pC/n for quartz and 140 pC/n for barium titanate For sensor 1 cm2 area and 1 mm thick a 10 g weight would give.23 and 14 mV respectively Can have different crystallographic structures to give thickness or longitudinal compression, transversal compression, thickness-shear and face shear Can also be had in polymeric films such as polyvinylidine flouride (PVDF) which are very thin and pliable 3

4. Piezoelectric Theory (cont’d) Can be seen as a parallel plate capacitor with voltage 4

5. Sensor Models Cp is sensor capacitance, Rp is sensor resistance 5

6. Signal Conditioning 6

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8. Gain Characteristics In voltage mode cable capacitance plays a part and motion artifact altering cable capacitance could be a problem 8

9. Alternate Signal Conditioning 9

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12. Ultrasonic Flowmeters Measure blood flow transcutaneously by injecting and receiving ultrasound signals Material can be lead zirconate titanate that has a very high energy conversion efficiency Crystal is one-half wavelength thick for maximum efficiency Produces a diffraction pattern just as an aperature does in optics Beam pattern has near field with concentrated energy and little spreading and depth d nf and spread an angle φ as shown 12

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14. Continuous-Wave Doppler Flowmeter When a target recedes from a fixed source that transmits sound, the frequency of the received sound is lowered because of the Doppler effect For small changes, fractional change in frequency = fractional change in velocity 14

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16. Technical Considerations Oscillator must have low output impedance to drive the crystal because it operates in resonance with and input impedance of about 100 Ω. 16